Effect of lowering the external Ca²⁺ concentration on EPSC amplitude. PF-EPSC amplitude (○) and CF-EPSC amplitude (●) were plotted as a function of the external Ca²⁺ concentration. The current amplitude was normalized to that at 2 mm external Ca²⁺. Values are presented as mean ± SEM from five measurements each. PF-EPSC amplitude was fit by a power relation,y = 0.30 ∗ x^m, where m = 1.75. CF-EPSC was fit by the Hill's equation, y = 1.0068 ∗x^m /(x^m + d′^m ), where m = 3.78 and d′ = 0.59. The relative half-saturation concentration d = 0.59 mm/2 mm = 0.29.